Short arc discharge lamp

ABSTRACT

A xenon arc discharge lamp for short pulse operation includes an elongated quartz tube and a pair of electrodes disposed along the longitudinal axis of the tube. The tube has a central axial bore of a relatively narrow cross section and two longitudinal gas reservoirs of relatively wider cross section disposed at the two outer end portions thereof. Two ends of the electrodes are spacedly disposed in the central bore to provide a short arc discharge gap and the axial bore is so dimensioned that the electrodes fit snugly therein. The electrodes or the internal surface of the axial bore include longitudinally disposed channels or grooves to permit the gas to flow between the arc gap and each reservoir. The discharge lamp of the present design advantageously relieves the high pressure that builds up at the arc discharge gap, replenishes gas into the arc gap as it is depleted by ionization and reduces the sputtering of the arcing surfaces of the electrodes.

United States-Patent Austad [541 SHORT ARC DISCHARGE LAMP [7-2] I Helge Austad, Lebanon, NJ.

[73] Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Jan. 21, 1971 Inventor:

Assignee:

Filed:

Appl. No.:

References Cited UNITED STATES PATENTS v I 12/1945 Suits "BIS/184x 10/1964 Beese ..3l3/267 Primary Examiner-Nathan Kaufman AttorneyC. Cornell Remen, Jr., Walter J. Baum,

2,456,896 12/1948 Slack'etal. ..-..31s/267x' 3,705,325 [451 Dec. 5, 1972 [57 ABSTRACT A xenon arc discharge lamp for short pulse operation includes an elongated quartz tube and a pair of electrodes disposed along the longitudinal axis of the tube. The tube has a central axial bore of a relatively narrow cross section and two longitudinal gas reservoirs of relatively wider cross section disposed at the two outer end portions thereof. Two ends of the electrodes are spacedly disposed in thecentral bore to provide a short are discharge gap and the axial bore is so dimensioned that the electrodes fit snugly therein. The electrodes or the internal surface of the axial bore include longitudinally disposed channels or grooves to permit the gas to flow between the arc gap and each reservoir. The discharge lamp of the present design advantageously relieves the high pressure that builds up at the arc discharge gap, replenishes gas into the arc gap as it is depleted by ionization and reduces the sputtering of the arcing surfaces of the electrodes.

7 Claims, 3 Drawing Figures 4 l6 aol 0 1/ l5 17 f 5 6 l lllllllfillllllllllllllllllllllllllllzlllblllllllllll 1 SHORT ARC DISCHARGE LAMP BACKGROUND OF THE INVENTION generate high current pulses with extremely short pulse duration and at relatively high repetition rates. One known form of a similar lamp may be found in US. Pat. No. 3,154,713, issued Oct. 27, 1964. Xenon arc dischargejlamps of this type,having generally spherical or elongated spherical gas chambers with electrodes disposed therein, have been found to have a number of shortcomings. Thus, for example, the attempts to operate such are discharge lamps at an extremely short pulse duration and high intensity have been met with a tendency for the arcing surface of the discharge electrodes to sputter badly during ionization of the gas and thereby more rapidly deplete the gassupply around the arc gap. This tends to shorten the life of the electrodes. This shortcoming is more critical for high intensity, short pulse duration lamps where, to shorten the pulse duration and rise and decay time, the area and volume of the arc gap are generally made smaller.

The small volume in the gap discharged at a high current, short duration, fast repetition rate, causes a gas expansion problem. At a low repetition rate, the gas has time to cool off between successive pulsing cycles. However, as the intensity and repetition rate increases, the cooling-off period is significantlyreduced and the rapid expansion of gas builds up an extremely high pressure against the wall of the tube which can result in an explosion and pose a serious safety hazard.

SUMMARY OF THE INVENTION It is accordingly the principleobject of the present invention to provide a high intensity, high current, short are discharge lamp of improved reliability and life, and increased power- According to the present invention, the objects of the invention are achieved by providing an arc discharge lamp which includes an elongated light transparent tube and a pair of electrodes disposed along the longitudinal axis of the tube. The tube is provided with arelatively narrow central axial bore and a relatively large gas reservoir disposed at each end of the bore. The two ends of the opposing electrodes are disposed in the central axial bore and spaced from each other to provide a short arc discharge gap. The electrodes are dimensioned to fit snugly into the bore and each of the electrodes or the quartz tube surrounding the elecdischarge gap is further relieved as the expanded gas escapes to the reservoirs through the passageways. Also advantageously, the gas reservoirsprolong the life of the discharge lamp by replenishingthe gas to the arc discharge gap as it is being depleted due to the ionization of thegas and minimizes the sputtering from the arcing surfaces of the electrodes.

These and other objects and features of the present invention will become more apparent from the following detailed description of the preferred embodiment of the present invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross sectional view of the arc discharge lamp structure of the present invention along the longitudinal axis;

FIG. 2 is a cross sectional end view of the electrode" and the surrounding quartz tube at the arc gap taken substantially along 2-2 of FIG. 1 showing a groove in the electrode; and

FIG. 3 shows a cross sectional view of the quartz tube and electrode similar to that of FIG. 2, with a groove provided in the axial bore of the quartz tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, as shown in FIG. 1, the arc discharge lamp, in accordance with the present invention, includes an elongated quartz tube 1 having centrally disposed axial bore 3 and a pair of longitudinal gas reservoir chambers 4 and 5 disposed at opposite ends of the bore. As shown in FIGS. 2 and 3, the axial bore is generally of a circular cross section, as are the gas reservoirs. The cross section of the bore is substantially narrower than that of the gas reservoirs.

A pair of electrodes 7 and 8, one of which may be a cathode and the other an anode, are disposed substantially along the longitudinal axis of the quartz tube with one end of each extending externally through an end of the tube for connection to a high intensity, pulsing source (not shown) and with the other ends 10, 11 disposed in the bore to form thearc discharge gap 9. Each of the electrodes may be made of pure tungsten rod with rounded arc ends 10, 11. In the alternative, the cathode or both the cathode and anode electrodes may be made of two sections, of which sections 12, 13 are made of pure tungsten or other material primarily to act as a high current conductor, and the other sec tions 14, 15 providing arcing sections brazed to the end sections 12, 13 respectively, and made of materials which provide good arcing surfaces. Thus, for example, the arcing section may include a small percentage of thoria, in the range of l to 5 percent, for the cathode. The arcin tip surface of the electrodes may also be coated with glass and a flame sprayed ceramic coating to limit sputtering.

For the brazing material of joining sections 16, 1 7, a material such as platinum, which is compatible with the thermal and electrical characteristic of the other two sections of the electrode, is used. In the aforementioned manner, a prime heat conduction path is provided by the high current conductor sections 12, 13 through the conductor-to-quartz end seals '18 and 19, which permits a large continuous metal path from the external electrodes to the internal end caps. An external heat sink (not shown) may also be employed.

The are discharge sections 14, of the electrodes are disposed substantially in the center of the longitudinal axial bore 3 as are the brazing junctions 16, 17 between the arc discharge sections 14, 15 and the conductor rods 12, 13. As illustrated in the drawings, the arc discharge sections 14, 15 fit snugly in the axial bore. The snug fit provides sufficient freedom of movement of the electrodes undergoing expansion and contraction during operation.

As shown in FIG. 2, the arc discharge section 14, and 15 (not shown), is provided with a groove or grooves 20 along the peripheral surface of the electrode parallel with the longitudinal axis. Alternatively, the internal quartz surface of the axial bore may be provided with a groove or flute 21, as shown in FIG. 3. The grooves or flutes provide passageways for the discharge gas, such as xenon, between the arc discharge gap 9 and the gas reservoirs 4 and 5, respectively.

Preferably, the internal diameter of the bore at the arc gap 9 should be dimensioned as narrowly as possible and the arcing space should be made as small as possible to provide discharge pulses of high intensity, short duration and fast repetition rate. For example, the cross section of the arc discharge gap may be close to the same dimension as the cross section of the electrodes as shown in FIG. 1 with some allowance for expansion with heat during operation. With the outer diameter of the quartz tube being uniform, the narrow axial bore at the center of the tube provides a thick tubular wall in the central section. The exact thickness of the bore is, however, a compromise between an adequate mechanical strength and minimum attenuation of light radiation output, which increases with quartz thickness. Preferably, the bore should be of precision tubing which, in addition to explosion protection, will provide other advantages such as excellent reproducibility and self-alignment of the electrodes.

The reservoir may be of a suitable design having sufficient volume to contain the discharge gas, as required by the arc discharge gap. The quartz tube may be constructed in a conventional manner with the precision central axial bore and two reservoirs communicating with each other. Air or other unwanted gas may be evacuated from the tube and the desired arc discharge gas, such as xenon, may be filled to a required level of pressure in a conventional manner via a tip 22 protruding from one of the reservoirs, after which the tip is sealed off.

In operation, the conducting gas is depleted by ionization. The depleted gas however, is replenished by the gas from the reservoirs. The reservoirs function also as the pressure relief means as the heated gas around the gap escapes to the reservoir and thus eliminates the explosion hazard and minimizes sputtering.

By way of an example, a high intensity, high current, short duration, high repetition rate, pulse arc lamp of the present invention may be constructed with the following specific characteristics: The cylindrical quartz tube is approximately 6 inches long and has an outer diameter of about one half inch with a central axial bore of about A inch diameter and 1.5 inches long. Other suitable materials such as sapphire, or ceramics such as a polycrystalline alumina known as Lucalox or yttrium oxide known as Yttralox, both made by the General Electric Co., may be used in place of quartz. In addition, the lamp may be made of a transparent center tube with metal or ceramic reservoirs. Each of the reservoirs has a diameter of about one half inch and extends about two inches along the length of the tube. The arc gap may be about three-thirty-seconds to oneeighth inch wide. The unwanted gas and air are initially evacuated from the gap and the reservoirs and are refilled with xenon gas to a level somewhat below atmospheric pressure, in the range of 450 to 550 mm. The lamp can also be filled with an alkali vapor or a mixture of such with other suitable gases.

The arc discharge lamp of the aforementioned construction when operated with a potential in the range of 20 to 40 volts applied across the electrodes, with 50 to amps and a repetition rate of about 100 pulses per second, provides a power output in the range of 0.5 watts average with a pulse width of about three microseconds. The pulse rise and decay time of the output pulse are in nanosecond range, and are thus negligible. A high direct voltage or rf pulse is required to start the arc. This may be in the order of 200 volts to 30,000 volts depending upon various parameters such as tube size and gas pressure.

It is to be understood that many other modifications of the arc discharge lamp may be made in the embodiment of the present invention without departing from the scope thereof as set forth in the appended claims.

What is claimed is:

1. An arc discharge lamp comprising:

a light transparent envelope having first and second gas reservoirs at opposite ends thereof,

a narrow central bore enclosed within a longitudinal portion of said transparent envelope and disposed between and communicating with said reservoirs,

first and second electrodes extending respectively through said opposite ends and disposed in said bore with the surfaces of said electrodes and surfaces of said bore being in a snug fit, the inner ends of said electrodes being spaced from each other to provide an arc discharge gap for said gas, and

a longitudinal passageway disposed in one of said surfaces along said electrodes and central bore, said passageway communicating with said reservoirs for permitting the gas to flow around said electrodes and between said are discharge gap and said first and second reservoirs.

2. The arc discharge lamp according to claim 1, wherein said passageway includes longitudinal grooves on the surfaces of said electrodes.

3. The are discharge lamp according to claim 1, wherein said passageway includes longitudinal grooves in the surfaces of said bore.

4. The are discharge lamp according to claim 1, wherein said reservoirs are of larger diameters than said bore.

5. The are discharge lamp according to claim 4, wherein the central portion of said transparent envelope around said bore is of greater thickness than at said reservoirs.

6. The are discharge lamp in accordance with claim 5, wherein said light transparent envelope is an elongated quartz tube, each of said first and second electrodes includes a high current conduction outer end section, an arc discharge inner end section and a joint therebetween having compatible thermal and electrical characteristics with said outer and inner-ends, each said outer end sectionbeing sealed to said quartz envelope.

7. The are discharge lamp according to claim 5, 5 wherein said gas is xenon. 

1. An arc discharge lamp comprising: a light transparent envelope having first and second gas reservoirs at opposite ends thereof, a narrow central bore enclosed within a longitudinal portion of said transparent envelope and disposed between and communicating with said reservoirs, first and second electrodes extending respectively through said opposite ends and disposed in said bore with the surfaces of said electrodes and surfaces of said bore being in a snug fit, the inner ends of said electrodes being spaced from each other to provide an arc discharge gap for said gas, and a longitudinal passageway disposed in one of said surfaces along said electrodes and central bore, said passageway communicating with said reservoirs for permitting the gas to flow around said electrodes and between said arc discharge gap and said first and second reservoirs.
 2. The arc discharge lamp according to claim 1, wherein said passageway includes longitudinal grooves on the surfaces of said electrodes.
 3. The arc discharge lamp according to claim 1, wherein said passageway includes longitudinal grooves in the surfaces of said bore.
 4. The arc discharge lamp according to claim 1, wherein said reservoirs are of larger diameters than said bore.
 5. The arc discharge lamp according to claim 4, wherein the central portion of said transparent envelope around said bore is of greater thickness than at said reservoirs.
 6. The arc discharge lamp in accordance with claim 5, wherein said light transparent envelope is an elongated quartz tube, each of said first and second electrodes includes a high current conduction outer end section, an arc discharge inner end section and a joint therebetween having compatible thermal and electrical characteristics with said outer and inner ends, each said outer end section being sealed to said quartz envelope.
 7. The arc discharge lamp according to claim 5, wherein said gas is xenon. 